It is your decision, but since you ask, my thinking is that if you were to catch COVID-19 at that party, then quite apart from what might happen to you if you did as such just from the COVID-19, depending on what effect it has on you even if there were not an operation pending - and I am not a clinician so this is may be just uniformed waffle - I am wondering if not only might your operation be cancelled in case you infect anybody else but also because having had the COVID-19 recently it might have weakened your body's resilience. There is also the Long COVID business.
Perhaps the isolate for 72 hours bit presumes that you have not got it at the start of the 72 hours.
A cake on your plate is better than ten cakes in a cake shop, to paraphrase a well-known non-vegan saying into a vegan one and add a bit of emphasis at the same time.
To add a positive note, how about sending a birthday card remotely by using the Papier facility. Either one of their stock designs or a design of your own.
www.papier.com/
The templates are good, the text is all set up as regards fonts and sizes so that you just need to change the wording, but if you want to change the font and colour and letter spacing you can often do so.
I have used that facility to send a card directly to someone and it worked well.
In fairness I have been in lockdown for 18 months now so I may have written the above with a bias to caution that some people might consider excessive or paranoid.
One other suggestion that might help. Imagine that you had come onto Gransnet and someone else had asked you for your advice to him or her in such a situation. What would you advise them to do?
I hope that this helps.
As I wrote, it is your decision, but if it were me, no way would I go. In fact I would self-isolate until the operation so as to minimise as best I could the possibility of the operation being cancelled.